"Intel's been using the 14nm lithography for a few years now. It has to be matured to the point where yields are very high. Where is this shortage coming from?"
1) Increased core counts mean a bigger die, a bigger die means less dies per wafer. Wafers per month per factory are pretty much fixed, so that means less product to sell when they add more cores. They went from 4 to 6 and now to 8, that's a significant impact to number of cpus they can crank out. Its not just the difference between die sizes either, if you increase your die size by 30%, you decrease the number of chips you can get by more then 30% due to the fact that bigger dies come with an increased chance that there will be a defect on the die. As well as adding more waste per wafer, rectangular dies dont fit perfectly on a circle wafter, there will be less complete dies that fit near the edges.
2) Moving their chipsets from 22nm to 14nm. This further eats into 14nm capacity because they assumed they would be on 10nm by now for the cpus, so they would have spare 14nm capacity for the chipsets. That is not the case. For every cpu they sell, they pretty much need to sell 1 chipset die, its a smaller die, but that still adds up to a lot of 14nm wafers going to chipsets instead of cpus.
3) Increased demand, if this is real demand, and not just the above 2 issues, then that further exasperates the issue.